Note: Descriptions are shown in the official language in which they were submitted.
216200~
Expandable styrene polymers comprising recycled material
The present invention relates to expandable styrene polymers
comprising recycled material which can be converted into foams
having an improved foam structure.
Plastics based on styrene and styrene-containing copolymers are
produced in large quantities and are used in many areas of
10 industry. Considerable importance is also attached to foamed
products.
The production and use of these products have been known for some
time and have been described in a variety of publications.
A problem which has become particularly important in recent years
is the recycling of used plastics.
In the case of polystyrene and polystyrene foams, various
20 recycling methods are known.
One possibility comprises dissolving used polystyrene in
monomeric styrene and re-polymerizing this solution by known
methods. Processes of this type are described, for example, in
US-A-5,269,948 and JP-A-56-60096.
Another possibility comprises using polystyrene foam or unfoamed
polystyrene waste for the extrusion of polystyrene foams.
30 This recycling material can be used either alone or as a mixture
with fresh polystyrene.
The polystyrene foams to be recycled should be compacted or
ground before processing. This can be carried out mechanically,
for example by pressing or grinding, but preferably by extrusion.
The production of foams from the recycled materials can then be
carried out directly, as usual, by steam treatment of the polymer
in the extruder with subsequent foaming to give boards or by
40 extrusion to give expandable minigranules. It is also possible to
produce minigranules which are impregnated with blowing agent in
a second step.
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All the known methods have the disadvantage that the foam
structure of the foamed styrene polymers produced using recycled
material is unsatisfactory. Cells are usually large and
irregular, and voids are formed.
These disadvantageous effects are caused, for example, by dirt
particles (or additives) in the recycled material which cannot be
removed completely even by melt filtration.
10 A further problem which adversely affects the use of
styrene-containing copolymers is the effect of the other polymer
components.
There has been no lack of attempts in the past to suppress these
adverse effects and to prepare expandable styrene polymers
containing recycled material whose foam structure and mechanical
property level are just as good as those made from fresh polymer.
Thus, nucleating agents, such as finely divided water or talc,
20 have been added to the styrene polymers. Although this resulted
in a more homogeneous foam structure, metering and homogeneous
distribution of these inorganic components are difficult. In
addition, in particular in the case of water, corrosion problems
during extrusion cannot be excluded.
It is an object of the present invention to prepare polystyrene
comprising recycled material which can be converted into foams
having a uniform foam structure and good mechanical properties
and which is simple to prepare.
We have found that, surprisingly, this object is achieved by
adding finely divided polyethylene wax to the expandable styrene
polymers comprising recycled material.
The present invention accordingly provides expandable styrene
polymers comprising recycled material to which homogeneously
distributed, finely divided polyethylene wax has been added.
The content of recycled polystyrene in the novel products is from
40 10 to 100% by weight, based on the total weight of the
polystyrene.
The finely divided polyethylene wax is added to the polystyrene
comprising recycled material in an amount of from 0.01 to 5%,
preferably from 0.05 to 0.5%, by weight, based on the total
amount of polystyrene.
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If smaller amounts of polyethylene wax are added, the action is
too weak, and if larger amounts are added disadvantages can occur
in the mechanical properties of resultant foams.
The fact that the object of the present invention can be achieved
by the addition of polyethylene wax was unforeseeable for the
person skilled in the art. Although the use of polyethylene wax
in the production of polystyrene foams is known, these products
are used, as described, for example, in GB-A-997 356, with the
10 aim of shortening the cooling time in particle foams.
The polyethylene wax used in accordance with the invention
generally has a mean molecular weight (number average) of from
2000 to 6000, preferably from 2000 to 4000, particularly
preferably from 2800 to 3500. An example of a suitable
polyethylene wax is Luwax~ supplied by ~ASF AG, which has a mean
molecular weight (number average) of 3000. The polyethylene wax
preferably has a mean particle size of from 5 to 50 ~m. If the
particles are too large, problems can occur with distribution in
20 the polymer, and if the products are too fine, dust nuisance can
arise.
The polyethylene wax is expediently added to the molten
polystyrene, preferably during extrusion.
In the production of extruder foam, the polyethylene wax can be
added during the extrusion step before foaming, and in the
production of polystyrene particle foam, it can be added during
the extrusion step to give minigranules. Polyethylene wax can
30 expediently be fed into the extruder together with the
polystyrene, but can also be introduced at any other point of the
extruder.
The polystyrene foam used for the production of the recycled
polystyrene must generally be compacted. This can be carried out
mechanically by pressing or grinding the recycled polystyrene
foam, but it is also possible to compact the recycled polystyrene
foam by extrusion.
40 In order to remove mechanical impurities which are insoluble in
the melt, the molten recycled polystyrene can advantageously be
subjected to melt filtration.
The novel expandable styrene polymers comprising recycled
material are expediently mixed with fresh polystyrene in order to
improve the properties of the resultant foams. This mixing is
2162~
usually carried out in the melt in an extruder. However, it is in
principle also possible to re-foam the recycled material alone.
The foaming is carried out by processes known per se.
Thus, it is possible to treat the molten polystyrene with steam
in the extruder and to foam the product directly thereafter to
give boards ~extruder foam technology).
10 However, it is also possible to granulate polystyrene after
extrusion, to impregnate these granules with blowing agent and
subsequently to foam the granules by known methods (particle foam
technology). The impregnation with the blowing agent can be
carried out while the granules are still in the extruder, in
which case the melt must be cooled rapidly in order to prevent
expansion of the particles (quenching).
However, the particles are usually impregnated in suspension. In
this case, the granules are suspended in a liquid, usually water,
20 in the presence of conventional auxiliaries and additives in a
pressure container, and the latter is rendered inert and heated
to a temperature above the softening point, but below the melting
point of the polymer. The blowing agent is injected at this
temperature. After cooling and decompression, the impregnated
granules are separated off, purified and dried, the latter
preferably at < 50 C in a stream of air.
This process is described, for example, in EP-A-343 473.
30 For the production of foams, the expandable styrene polymers are
expanded in a known manner by heating to temperatures above their
softening point, for example by means of hot air or preferably by
means of steam. After cooling and, if desired, interim storage,
the foam beads can be re-expanded by further heating. Finally,
they can be welded in a known manner in non-gas-tight molds to
give moldings.
Further details on conventional impregnation, extrusion and
foaming processes are given, for example, in Polystyrene, edited
40 by R. Vieweg and G. Daumiller, Carl-Hanser-Verlag, Munich, 1969.
The novel polystyrene foams are distinguished from other
polystyrene foams comprising recycled material through a more
uniform foam structure. Even a small amount of contamination by
other polymers and additives in the recycled material, which
cannot be removed by conventional purification methods, such as
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melt filtration, do not have any disadvantageous effect on the
foam structure of the novel products.
This allows even foamed and unfoamed polystyrene waste produced
outside the actual production process, which varies in material
characteristics and in some cases also has different additives,
to be re-converted into high-quality foams by a very simple
process.
lO The polyethylene wax used in accordance with the invention is
readily available. It has no disadvantageous effects on the
properties and the processing behavior of the foams.
In addition, molecular weight degradation of the recycled
polystyrene, measured as the viscosity number, is significantly
reduced, ie. the polymer suffers less damage during extrusion.
The invention is described in greater detail by means of the
examples below:
Examples 1 - 3 (comparison)
Granulated recycled polystyrene (produced using an EREMA
extruder, insoluble content 0.12% by weight, viscosity number
65 ml/g), if desired blended with pure polystyrene having a
viscosity number of 75 ml/g (polystyrene VPT from BAS~ AG), was
extruded in a Werner und Pfleiderer ZSK 53 extruder with
introduction of 5% by weight of pentane and subsequently
granulated under water to a size of about 1.4 x 1.5 mm.
The granules were pre-foamed by means of a pressureless foaming
box (Rauscher system) and converted into board-shaped moldings
after 12 hours.
Examples 4 - 12
The procedure was as in Examples 1 - 3, but a polyethylene wax
having a molecular weight of about 3000 and a particle size of
about 10 ~Im (Luwax~ AF 31 from BASF AG) was added during the
40 extrusion.
Examples 13 - 15 ( comparison)
The procedure was as in Examples 1 - 3, but the blowing agent was
not added in the extruder, but instead the granules were
suspended in water with a Pickering stabilizer based on magnesium
pyrophosphate, transferred into a nitrogen-flushed pressure
21~2~05
reactor, impregnated with 7% by weight of pentane and foamed as
described in Examples 1 - 3.
Examples 16 - 18
The procedure was as in Examples 13 - 15, but a polyethylene wax
having a molecular weight of about 3000 and a particle size of
about 10 llm (Luwax~ AF 31 from BASF AG) was added during the
extrusion.
The precise mixing ratios of the starting materials and the
properties of the foams are shown in Table 1.
The viscosity number (0.5~ in toluene) was measured in accordance
with DIN 53 726, and the foam structure and cell count were
assessed visually.
Table 1
ExampleProportion of Amount of Viscosity number Foam structureCell count
recycled polystyrene polyethylene wax [mg/g] [Cells/mm]
[% by weight] [~ by weight]
1 0 0 68 good 8
2 50 0 65 satisfactory 7
3 100 0 62 poor S
4 0 0 67 satisfactory 8
0 64 satisfactory 7
6 100 0 62 satisfactory 5
7 0 0.05 68 good 8
8 50 0.05 65 satisfactory 7
9 100 0.05 62 satisfactory 7
0 0.1 69 good 8 ~3
11 50 0.1 67 good 7 ~-
12 100 0.1 64 satisfactory 7 ~3
13 0 0.3 70 good 9 ~3
14 50 0.3 68 good 8
100 0.3 65 good 7
16 0 0.1 68 good 8
17 50 0.1 67 good 8
18 100 0.1 64 satisfactory 7
